This invention relates to a non-aqueous liquid electrolyte secondary cell, and more particularly to a non-aqueous liquid electrolyte secondary cell having a lithium double oxide and a material which is capable of occluding and releasing lithium ions or has lithium ion occluding and releasing properties incorporated therein so as to act as a positive active material and a negative active material, respectively.
There has been conventionally known a non-aqueous liquid electrolyte secondary cell in the art which includes a positive electrode and a negative electrode wherein the positive electrode has a positive active material layer containing a positive active material consisting of a lithium double oxide formed on a positive collector and the negative electrode has a negative active material layer containing a negative active material having lithium ion occluding and releasing properties formed on a negative collector. Such a non-aqueous liquid electrolyte secondary cell has been extensively used for a potable electric/electronic equipment such as a VTR camera, a note-type personal computer, a portable telephone or the like, because it is increased in energy density. Active materials incorporated in the non-aqueous liquid electrolyte secondary cell are chemically active, so that a non-aqueous liquid electrode of the cell is deteriorated in performance when water intrudes thereinto. In order to eliminate the disadvantage, the non-aqueous liquid electrolyte secondary cell is constructed into a hermetically sealed structure. Unfortunately, such a hermetically sealed structure causes an internal pressure of the cell to be increased due to gas generated by decomposition of the electrolyte, leading to bursting of a cell case of the non-aqueous liquid electrolyte secondary cell, resulting in damage to a peripheral equipment, when it falls into a supercharged state due to any trouble or failure of a charging circuit. In order to solve such a problem, it was proposed that the cell case of the non-aqueous liquid electrolyte secondary cell is provided therein with a current breaking device such as a pressure switch for interrupting electrical connection between the electrodes and cell terminals when the internal pressure is increased, as detailedly discussed in U.S. Pat. No. 5,567,539, which corresponds to Japanese Patent Application Laid-Open Publication No. 102331/1996 (8-102331), and the like.
Nevertheless, arrangement of such a current breaking device often leads to generation of heat in the cell case which causes a rapid increase in temperature of the cell rather than generation of gas therein in the supercharged state, before an increase in internal pressure of the cell due to generation of gas by decomposition of the electrolyte permits operation of the current breaking device to be carried out. In the worst case, the heat generation often causes uselessness of the current breaking device, leading to breakage or explosion of the cell. In order to avoid such a situation, techniques of incorporating an additive such as lithium carbonate or lithium oxalate in the positive active material were proposed as disclosed in Japanese Patent Application Laid-Open Publication No. 329269/1992 (4-329269) and U.S. Pat. No. 5,427,875 which corresponds to Japanese Patent Application Laid-Open Publication No. 328278/1992 (4-328278). Incorporation of the additive in the positive active material permits carbon dioxide to be generated due to electrochemical decomposition of lithium carbonate or lithium oxalate contained in the positive active material, when the cell is subject to supercharging. It would be considered that carbon dioxide thus generated acts to not only restrain any abnormal reaction which causes heat generation leading to a rapid increase in temperature of the cell, but increase an internal pressure of the cell to ensure positive actuation of the current breaking device. However, actually, the additive thus incorporated in the positive active material fails to fully restrain such a temperature increase of the cell depending on supercharged conditions thereof. In particular, in techniques disclosed in Japanese Patent Application Laid-Open 329269/1992 described above, in order to effectively decompose lithium oxalate, the positive active material is mixed with lithium carbonate to prepare a mixture, which is then subject to a heat treatment, to thereby permit lithium carbonate to be contained in the positive active material. Unfortunately, this causes particles of the positive active material to be increased in size, leading to a decrease in specific surface area of the active material, resulting in current density being increased, so that the non-aqueous liquid electrolyte secondary cell may be deteriorated in both high-rate discharge characteristics and low-temperature discharge characteristics.
In order to solve the problem, it was proposed to incorporate an additive such as manganese carbonate, cobalt carbonate, nickel carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, magnesium carbonate, calcium carbonate, barium carbonate or the like in the positive active material layer, as disclosed in Japanese Patent Application Laid-Open Publication No. 338323/1994 (6-338323) and U.S. Pat. No. 5,567,539, which corresponds to Japanese Patent Application Laid-Open Publication No. 102331/1996 (8-102331). Use of such an additive solves the problem described above.
Nevertheless, incorporation of the additive in the positive active material very rarely causes a failure in actuation of the current breaking device when an internal pressure of the cell is increased due to generation of gas by decomposition of the electrolyte owing to overcharging of the cell, even if it restrains a rapid increase in temperature of the cell. This would be due to materials for components for the current breaking device, assembling of the device and quality of welding for joining between the components. Such a failure in actuation of the current breaking device further promotes supercharging of the cell to increase a voltage across the cell, to thereby increase a charging current correspondingly, when the additive for restraining an increase in temperature of the cell as described above is incorporated in the positive active material. Such an increase in rate of increase in charging current causes abnormal or excessive heat generation, so that a rate at which the electrolyte is decomposed is increased correspondingly. This possibly causes early bursting or explosion of the cell when it is not provided with a relief valve for releasing an internal pressure of the cell. When the relief valve is arranged, operation of the valve reduces an internal pressure of the cell; however, it possibly causes an electrolyte to leak to an outside of the cell, leading to damage to a peripheral equipment.